1. Field of the Invention
The present invention relates to flexible vessels for transporting fluent cargoes through liquid.
2. Description of the Related Art
Vessels of this nature are known and can take the form of closed flexible tubular or envelope structures, generally of a synthetic rubberised fabric. Such vessels are used to transport fluent cargoes having a specific gravity less than that of the liquid in which they are being transported. Ordinarily, the fluent cargo has a specific gravity less than that of seawater. When laden, the vessel is virtually completely submerged and buoyancy is supplied by the cargo. To move such laden vessels, they are towed from one end by for example a tug boat.
One such vessel is known from GB-A-0 883 813 wherein the vessel is made in a tubular shape with a streamlined bow. The vessel is towed by way of a rope that is attached to the bow of the vessel.
Such laden vessels have intrinsic stability problems, particularly where the vessel is used in anything other than calm seawater conditions. For example, waves can set up undesirable oscillations in the fluent cargo. The cargo will in this regard have a natural frequency which if coincidental with an applied wave frequency, can lead to resonance. The resulting magnification of the oscillation of the fluent cargo is clearly to be avoided otherwise damage and rupture of the vessel can occur. In addition, the exterior profile of the vessel can be distorted from its most streamlined and drag-free configuration such that the vessel requires an increased towing force to maintain speed.
In this connection, measures have been proposed for reducing the effects of resonance within the vessel itself, for example by providing internal stabilisers in the form of fabric diaphragms within the vessel interior, such diaphragms having holes or perforations to control the flow of cargo. Also, a vibration absorber within the vessel has been proposed comprising an inner tube of fabric so filled as to resonate at the forcing frequency of the waves. However, such measures have only a limited effect and introduce other problems.
Another problem arises in that such laden vessels exhibit yaw. This is where the vessel moves off the preferred towing direction in an uncontrolled fashion so that the vessel can become extremely difficult to control.
In this connection, it has been common practice to mount a stabiliser in the form of a specially configured end, skirt or drogue at the rear end of the vessel to reduce the tendency of the vessel to yaw undesirably. From GB-A-0 846 359, for example, in order to improve stability, the containers of this document are provided with large fins to attempt to prevent yaw and roll. However, these in practice do not wholly solve the problem of yaw and in any case the extra drag caused thereby significantly increases the towing forces required to move the vessel. Indeed, such forces can be doubled. Clearly this has a disadvantageous effect on fuel efficiency of the towing means and thus overall running costs. In addition, this has proved a cap on the maximum practical cargo load that can be transported, typically in the range of 1,000 to 3,000 m.sup.2. Attempts at transporting higher loads have failed.
Another problem arises in that such laden vessels exhibit instability in twist, due to shear forces created by breaking waves, which leads to capsize in bow and quarter seas in particular.
It is thus an object of the present invention to provide an apparatus for transporting fluent cargoes which has an improved stability without significant sacrifices to streamlining efficiency or vessel size.